The present invention relates to a fuel property determination method and apparatus for determining property of fuel supplied to internal combustion engines.
In gasoline engines mounted on vehicles, gasoline fuel is injected in intake ports of the engine by fuel injectors mounted on an intake pipe of the engine. Most of the injected fuel is directly sucked into cylinders of the engine. However, a part of the injected fuel remains on the inner peripheral walls of the intake port or on the surface of intake valves for a moment, and then gradually evaporates and enters the cylinders. Thus, the amount of fuel sucked into each cylinder varies with the amount of evaporation of fuel remaining on the inner peripheral wall or the like as the fuel wet.
The amount of evaporation, that is, speed of evaporation, depends on fuel property. The fuel property varies even among the same type of fuel and among fuel producers. It is also varied in dependence on season and sales area, even if the fuel is of the same producer's. It is therefore necessary to accurately determine the fuel property so that the air-fuel ratio of air-fuel mixture supplied to the engine may be accurately controlled in consideration of the amount of evaporation of fuel.
It is proposed to detect the fuel property by a fuel property sensor provided in a fuel tank. The fuel property sensor, however, adds cost.
It is also proposed in JP-A-5-65838 to measure a period required to complete engine cranking by increasing the amount of fuel at the time of engine starting, and to determine the fuel property based on the measured engine starting period. However, the engine starting period varies with engine coolant temperature and other factors other than the fuel property, thus disabling accurate determination of the fuel property from the measured engine starting period.